This invention relates to electromagnetic interference (EMI) and radio frequency interference (RFI) shielding media and specifically to omnidirectional EMI/RFI shielding media and methods for fabricating the same.
The terms EMI and RFI refer to the impairment of performance of an electronic system or subsystem by unwanted electromagnetic disturbances. Recently, EMI and RFI have been referred to generically as EMI. Throughout this application, reference will only be made to EMI, but such reference includes EMI and RFI.
Electromagnetic compatibility is achieved by reducing the EMI below the level that disrupts proper operation of the electronic system. This compatability is generally achieved by means of various devices, such as line filters and equipment shields. Any barrier placed between any emitter of EMI and an electronic system which is suceptible to disruption that diminishes the strength of the interference can be regarded as an EMI shield. The shielding effectiveness is the measure of the ability of that media to control radiated electromagnetic energy. The standard unit for measurement of shielding effectiveness is the decibel (dB).
The losses in field strength from a shielding media are a function of a number of parameters, including the barrier material (permeability, conductivity and thickness), frequency and distance from the EMI source to the shield. In most shielding applications, shielding effectiveness below 20 dB is considered only minimal shielding, 20 dB to 80 dB covers the normally acceptable shielding range and 80 dB to 120 dB is above the average shielding. Above 120 dB shielding effectiveness is difficult to achieve often requiring the use of special designs, materials and finishes and difficult to confirm by measurement.
A number of various shielding media is known in the prior art. One of the most commonly employed shielding media for covering openings in an enclosure, which either houses the emitter or potential receptor of EMI, is a metallic honeycomb structure, commonly made from steel, brass or aluminum. The honeycomb structure is commonly fabricated by applying adhesive or glue at spaced apart intervals to a plurality of straight metal strips. When the adhesive or glue has set, the metal strips, which are readily bendable are pulled apart in a direction perpendicular to the faces of adhesion. By spacing the adhered or glued sections away from each other, a distance equal to the length along the strip of the glued or adhered section, a generally hexagonal honeycomb is generated.
A principal disadvantage of the above-referenced honeycomb structure lies in the fact that immediately adjacent strips are electrically insulted from each other by virtue of the fact that the adhesive or glue is essentially non-conductive. Accordingly, the honeycomb structure exhibits unidirectional electrical conductivity in generally parallel paths along each metal strip. While these electrically conductive pathways provide excellent conductivity in the direction of the long dimension of the strips, they provide virtually no electrical conductivity in pathways normal to the plain of each conductive strip.
Most EMI is well-known to be omnidirectional. By virtue of the fact that EMI shielding is achieved by channelling the interfering electromagnetic radiation along the pathways of conductivity to a ground station, the shielding effect of the abovereference honeycomb structure is unidirectional and, therefore, functions poorly with omnidirectional EMI at those frequencies which are most troublesome.
Omnidirectional EMI panels or media are known in the prior art. One such panel is shown and described in U.S. Pat. No. 3,821,463 and comprises essentially a pair of honeycomb panels as above-referenced in fact-to-face engagement with each other with the continuously conductive metal strips of one panel oriented at right angles to the continuously conductive strips of the other panel. Other omnidirectional panels are known in which the honeycomb structure above-referenced and a frame surrounding the structure are nickel plated in a non-electrical plating operation. Such a honeycomb structure may also be plated with other metals such as cadmium, tin, copper and steel. The plating process to generate such a shielding media is both time-consuming and expensive particularly in the case of nickel plating. Obviously, in the case of the structure shown in U.S. Pat. No. 3,821,463, the expense of two honeycomb panels must be borne and in addition thereto, there is an increase in both weight and thickness of the panel and an increase of the pressure drop in air flowing through the shield over a similar shield having only one thickness of honeycomb.
It is desirable to employ the honeycomb structure described above, made from aluminum because of corrosion resistance, light weight and ease and low cost of manufacture of the honeycomb structure itself. Accordingly, it is among the objects and advantages of the present invention to provide an EMI shielding media which is both omni-directional and inexpensive to manufacture, particularly of aluminum which is corrosion resistant. Additionally, it is among the objects and advantages of the present invention to provide an EMI shielding media which generates the lowest possible pressure drop or impediment to the passage of cooling air.
In accordance with the present invention, a light-weight omnidirectional EMI shielding media is fabricated by preparing an open latticwork structure of electrically conductive members, preferrably a honeycomb, in the form abovereferenced, by applying adhesive or glue to adjacent, elongated, relatively electrically conductive members or strips at spaced-apart intervals and pulling the member apart at right angles to the long axis of the member or strips to generate a latticework having transverse openings with the edges of the strips defining generally planar faces. Thereafter, at least one face of the media generated thereby is flame sprayed with a relatively electrically conductive powder to generate electrically conductive bridges between adjacent strips at the site of mechanical joinder. The electrical bridges thereby provide conductive pathways normal to the long dimension of the electrically conductive strips.